Development and structures of the venous pole of the heart

In the past, our interpretations of cardiac development depended on analysis of serially sectioned embryos, supported by three-dimensional reconstructions. It was not possible, using these techniques, to trace the fate of the various embryonic building blocks. This has all changed with the advent of the new techniques in molecular biology. Combining our experience with these new techniques and our previous studies using the classic approach, we have reviewed how the recent advances clarify controversies that still exist concerning the development of the venous pole. The arguments devolve on whether the pulmonary vein is itself a new development or whether its primordium is derived from the systemic venous tributaries, the so-called sinus venosus. The new techniques show that, rather than developing in the form of a segmented tube, the heart is built up by addition of material to both its arterial and venous poles. At no stage is it possible to recognize a discrete part of the tube that can be identified as the sinus venosus. The confluence of the systemic venous tributaries does not become recognizable as a discrete anatomic entity until compartmented into the newly formed right atrium concomitant with formation of the venous valves. The new molecular techniques show that the pulmonary vein is a new structure, anatomically and developmentally, that is derived from mediastinal myocardium. It gains its connection to the morphologically left atrium between the right- and left-sided systemic venous tributaries.